Method for determining a characteristics difference between fluids

ABSTRACT

A characteristic difference between first and second liquids is measured using a surface having a monolayer of a voltage sensitive chromophore that is covalently bound to the surface. The first liquid is brought into contact with the surface and it is irradiated with actinic radiation to measure a first fluorescence emission spectrum. The second liquid is also brought into contact with the surface and it is irradiated with actinic radiation to measure a second fluorescence emission spectrum. The first and second fluorescence emission spectra are compared to characterize a difference between the first and second fluids.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (Docket K002020), entitled: “Method formeasuring solid-liquid interfacial potentials,” by A. Wexler et al.; tocommonly assigned, co-pending U.S. patent application Ser. No. ______(Docket K002101), entitled: “Method for characterizing a liquid,” by A.Wexler et al.; and to commonly assigned, co-pending U.S. patentapplication Ser. No. ______ (Docket K002103), entitled: “Surfaceevaluation system using voltage sensitive chromophore,” by A. Wexler etal., each of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the determination of characteristicdifferences between liquids by measuring differences between thefluorescence emission spectra of a voltage sensitive chromophore affixedto a surface in contact with the liquids.

BACKGROUND OF THE INVENTION

Voltage sensitive dyes (VSD), also known as potentiometric dyes orvoltage sensitive chromophores, have been used to determine membranepotentials for biological cells. Such dyes have been designed to becomeembedded within a cell membrane orthogonal to the membrane surface wherethey are then exposed to the membrane electric field. U.S. PatentApplication Publication 2010/0074847 to Madden et al., describes the useof chromophore probes for optical imaging of biological materials. Theuse of voltage sensitive dyes, is also discussed by L. Loew in anarticle titled “Potentiometric dyes: Imaging electrical activity in cellmembranes” (Pure & Appl. Chem., Vol. 68, pp. 1405-1409, 1996). The useis very similar to that described U. S. Patent Application Publication2010/0074847.

The absorbance or fluorescence emission of a VSD can be measured for themembrane in both the polarized state and un-polarized state. The shiftin the resulting spectrum gives the electrical potential differencebetween the two states.

Two types of VSDs, which vary in their response mechanism, are known.What are considered “slow” VSDs partition within the cell, and theirfluorescence intensity is a Nernstian-concentration-dependent response.What are considered “fast” VSD's do not depend on partitioning andinstead respond to the electric field directly via the Stark effect.

Experimental strategies for measuring electric field strength(intensity) at charged solid surfaces using a fluorescent dye as a probeparticularly in bulk behavior are described by J. Pope et al. in anarticle entitled “Measurement of electric fields at rough metal surfacesby electrochromism of fluorescent probe molecules embedded inself-assembled monolayers” (J. Am. Chem. Soc., Vol. 114, pp.10085-10086, 1992), as well as by J. Pope et al. in an article entitled“Measurements of the potential dependence of electric field magnitudesat an electrode using fluorescent probes in a self-assembled monolayer”(J. Electroanalytical Chem., Vol. 498, pp. 75-86, 2001).

U.S. Pat. No. 5,156,918 No. (Marks et al.) describes the use of apoly(phenylene ether) to which a pyridine-terminated chromophore isattached and the resulting nonlinear optical material can be covalentlyattached to solid surfaces as a monolayer for various purposes.

Surface potential is a key parameter in colloidal and biologicalsciences governing interactions between materials such as the attractiveor repulsive interaction between materials. Thus, adhesion forcesbetween particles or polymers and a surface that affects particlepatterning are directly influenced by surface potentials. Such particlesand polymers cover a wide range of materials ranging from biological toinorganic materials. Currently there are two prominent methods todetermine surface potentials. However, each of these methods create atleast one problem. For example, the method of using streaming potentialsdescribed by H. Xie et al. in the article “Zeta potential ofion-conductive membranes by streaming current measurements” (Langmuir,Vol. 27, pp. 4721-4727, 2011) requires expensive equipment and themethod is damaging to tested samples. Moreover, the use of atomic forcemicroscopy as described by S. Li et al. in the article “Excludingcontact electrification in surface potential measurement using Kelvinprobe force microscopy” (ACS Nano, Vol. 10, pp. 2528-2535, 2016) isrestricted to very small areas and requires long scanning times.

There remains a need for less burdensome and more versatile methods fordetermining solid-liquid interface electrical field intensities, and fordetermining the characteristics of liquids.

SUMMARY OF THE INVENTION

The present invention represents a method for determining acharacteristic difference between first and second fluids, including:

providing a surface having a reactive carbocyclic aromatic linking groupcovalently attached thereon;

providing a voltage sensitive chromophore precursor including a psubstituted dialkylamino aryl group that is conjugatively linked to aterminal N containing heterocyclic aromatic group;

reacting the voltage sensitive chromophore precursor with the reactivecarbocyclic aromatic linking group that is covalently attached to thesurface to form a monolayer of a voltage sensitive chromophore that iscovalently bound to the surface;

bringing the first fluid into contact with the monolayer of thecovalently bound voltage sensitive chromophore;

irradiating the monolayer of the covalently bound voltage sensitivechromophore with actinic radiation while it is in contact with the firstfluid and measuring a first fluorescence emission spectrum;

bringing the second fluid into contact with the monolayer of thecovalently bound voltage sensitive chromophore;

irradiating the monolayer of the covalently bound voltage sensitivechromophore with actinic radiation while it is in contact with thesecond fluid and measuring a second fluorescence emission spectrum; and

comparing the first and second fluorescence emission spectra tocharacterize a difference between the first and second fluids.

This invention has the advantage that it provides a method forcharacterizing a difference between first and second fluids using asensor that detects a shift in the fluorescence emission spectrum of avoltage sensitive chromophore.

It has the additional advantage that an unknown liquid can be identifiedby determining whether its fluorescence emission spectrum matches thatof a known liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for measuring an interfacial electricfield intensity according to an exemplary embodiment;

FIG. 2A illustrates an exemplary first reaction useful in the method ofFIG. 1;

FIG. 2B illustrates an exemplary second reaction useful in the method ofFIG. 1;

FIG. 2C illustrates an exemplary third reaction useful in the method ofFIG. 1;

FIG. 3A is a graph showing a Stark shift measured using the method ofFIG. 1 for an exemplary voltage sensitive chromophore with a quartzsurface and water;

FIG. 3B is a graph showing a Stark shift measured using the method ofFIG. 1 for an exemplary voltage sensitive chromophore with a quartzsurface and methanol;

FIG. 3C is a graph showing a Stark shift measured using the method ofFIG. 1 for an exemplary voltage sensitive chromophore with a quartzsurface and acetone;

FIG. 4 is a schematic showing a voltage sensitive chromophore bonded toa silica surface;

FIG. 5A is a schematic showing a monolayer of voltage sensitivechromophore where the dipole P is flipped upon excitation with actinicradiation in the presence of an interfacial electric field;

FIG. 5B is an energy diagram illustrating the energy differenceresulting from an interfacial electric field;

FIG. 6 is a graph illustrating a relationship between the dielectricconstant of the liquid and the resulting Stark shift;

FIG. 7 is a flowchart of a method for measuring a liquid characteristicaccording to an exemplary embodiment;

FIG. 8 is a flowchart of a method for determining characteristicdifferences between two liquids according to an exemplary embodiment;

FIG. 9 is a graph illustrating a method for comparing two fluorescenceemission spectra at first and second wavelengths; and

FIG. 10 illustrates an exemplary surface evaluation system forevaluating a surface in accordance with an exemplary embodiment.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion is directed to various embodiments of thepresent invention and while some embodiments can be desirable forspecific uses, the disclosed embodiments should not be interpreted orotherwise considered to limit the scope of the present invention, asclaimed below. In addition, one skilled in the art will understand thatthe following disclosure has broader application than is explicitlydescribed in the discussion of any embodiment.

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. The useof singular or plural in referring to the “method” or “methods” and thelike is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

Each term that is not explicitly defined in the present application isto be understood to have a meaning that is commonly accepted by thoseskilled in the art. If the construction of a term would render itmeaningless or essentially meaningless in its context, the termdefinition should be taken from a standard dictionary.

The use of numerical values in the various ranges specified herein,unless otherwise expressly indicated otherwise, are considered to beapproximations as though the minimum and maximum values within thestated ranges were both preceded by the word “about”. In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as the values within the ranges.In addition, the disclosure of these ranges is intended as a continuousrange including every value between the minimum and maximum values.

When a solid is immersed in a fluid, an electric field is generated atthe interface. This interfacial electric field can also be characterizedby a corresponding interfacial potential difference. In accordance withembodiments of the invention, the interfacial electric field (or thecorresponding interfacial potential difference) can be quantified bymeasuring a wavelength shift of the fluorescence emission spectrumbetween a fast VSD present in the liquid and a monolayer of the VSDaffixed to a solid surface in contact with the same liquid (or via aprobe with bound VSD immersed in the liquid). Given the same solventshell structure and reorganization on excitation for bulk and surfaceVSD bound at the interface, the fluorescence wavelength shift uponexcitation is due to the extra energy needed to flip the chromophoredipole in the interfacial electric field. Chromophore dipole moments fora series of voltage sensitive dyes are available in the literature (forexample, see P. Fromherz, “Monopole-dipole model for symmetricalsolvatochromism of hemicyanine Dyes,” J. Phys. Chem., Vol. 99, pp.7188-7192, 1995). The solvatochromic shift of the VSD is cancelled outsince both fluorescence spectra (that is, the bound VSD monolayer anddissolved VSD in the liquid) are recorded in the same solvent.

In various embodiments, the shift in fluorescence emission spectrum of abound VSD at a solid/liquid interface can be used to perform variousfunctions such as calculating the electric field strength at theinterface between a liquid and a surface, identifying or characterizingliquid, determining characteristic differences between a plurality ofliquids, and evaluating surface characteristics/uniformity.

FIG. 1 shows a flowchart of a method for determining an interfacialelectric field intensity 175 in accordance with an exemplary embodiment.A series of reactions is used to add a monolayer of a voltage sensitivechromophore (i.e., a voltage sensitive dye (VSD)) to a surface 100. Thesurface 100 can be a non-biological surface such as a silica surface ora polymer surface. In a first reaction 110, a linking group 105including a chromophore component is covalently attached to the surface100. The linking group 105 can also be referred to as a “reactivelinking molecule.” In an exemplary embodiment, the first reaction 110 isperformed by: (a) providing a solution including the linking group; (b)providing a surface capable of reacting with the linking group; and (c)bringing the solution into contact with the surface, thereby reactingthe linking group with the surface to attach the linking group to thesurface with a covalent linkage.

FIG. 2A illustrates an exemplary first reaction 110 that can be used inaccordance with the present invention. In this example, the surface 100is a silica surface, and the linking group 105 includes a chromophorecomponent 145 and a coupling agent 147. The coupling agent 147 isadapted to couple the linking group 105 to the surface 100. Theillustrated chromophore component 145 includes a reactive carbocyclicaromatic linking group (in this case a benzyl halide group), and thecoupling agent 147 has a trichlorosilane coupling group. This couplinggroup is known to couple the silane to a silica surface that has silanol(Si—OH) groups on it, forming Si—O—Si bonds. The resulting surface withlinking group 115 includes the chromophore component 145 (i.e., thereactive carbocyclic aromatic linking group) which is attached to thesilica surface 100 by a covalent linkage 112.

For other types of surfaces 100, linking groups 105 having otherappropriate coupling agents 147 can be used. For example, polymers havecarboxylic acid groups (—COOH) that can react with an alcohol (C—OH)coupling group to form an ester (—COOC) linkage. Surfaces withcarboxylic acid groups can also couple with an amino (—NH2) couplinggroup to form an amide (—CONH—) linkage. Alternatively, carboxylic acidgroups on surfaces can be coupled to amine coupling groups to form anamino acid linkage. In other examples, isocyanate coupling groups (—NCO)can be used to form a linkage to surfaces with carboxylic acids,alcohols, and amines. Other reactive functional groups include aromaticchloromethyl, amide, hydrazide, aldehyde, hydroxyl, thiol and epoxy.Clearly, similar reactions can be used for many types of surfaces 100,as long as the surfaces 100 have functional groups, or can be modifiedwith functional groups, that will react with the coupling agent 147 thatis included in the linking group 105.

Returning to a discussion of FIG. 1, in a second reaction 125, a voltagesensitive chromophore precursor 120 is reacted with the surface withlinking group 115 to form a monolayer of the voltage sensitivechromophore that is covalently bound to the surface (i.e., voltagesensitive chromophore surface 130). The voltage sensitive chromophoresurface 130 can also be referred to as the “sensor surface” because themonolayer of the voltage sensitive chromophore provides the function ofa sensor for sensing properties of the surface, such as the interfacialelectric field, or properties of a liquid that is brought into contactwith the surface.

In an exemplary embodiment, the voltage sensitive chromophore precursor120 includes a p-substituted dialkylamino aryl group 180 that is linkedto a terminal N-containing heterocyclic aromatic group 185 with aconjugate linkage 182 as illustrated in FIG. 2B. In this example, theN-containing heterocyclic aromatic group 185 is a pyridinyl group (apyridinyl group is a radical derived from pyridine). In the secondreaction 125, the chromophore component 145 (i.e., the reactivecarbocyclic aromatic linking group) reacts with the terminalN-containing heterocyclic aromatic group 185 of the voltage sensitivechromophore precursor 120 to form the voltage sensitive chromophore 152.The voltage sensitive chromophore 152 can also be referred to as avoltage sensitive dye (VSD). The voltage sensitive chromophore 152 hasan associated dipole moment, and is “voltage sensitive” in that itsfluorescence emission spectrum (or its absorbance spectrum) changes whenit is placed in an electric field, that is, in a region where thevoltage (i.e., potential) is changing.

The parts of the voltage sensitive chromophore precursor 120 (i.e., thep-substituted dialkylamino aryl group 180 and the N-containingheterocyclic aromatic group 185) are conjugatively linked and aredesigned to be reacted with the chromophore component 145 to yield thevoltage sensitive chromophore 152. One skilled in the art will recognizethat a conjugatively-linked system (also known as a “conjugated system”)is a system of bonded atoms including pi orbitals that have beendelocalized through bonding. The pi electrons in these orbitals aredelocalized over the atoms that participate in the pi orbital bonding.This normally lowers the total energy of the system, and is favored insuch bonding as alternating single and double bonds, certain functionalgroups attached to aromatic rings, alternating single and double bondsattached to aromatic rings, etc.

In alternate embodiments, other types of molecules can be chosen for thetwo chromophore precursors (i.e., chromophore component 145 and thevoltage sensitive chromophore precursor 120), depending on theparticular voltage sensitive chromophore that is used (e.g., any of thevoltage sensitive chromophores in the aforementioned article by P.Fromherz, entitled “Monopole-dipole model for symmetricalsolvatochromism of hemicyanine Dyes”). Typically, the resulting voltagesensitive chromophore is a conjugated system, and is formed from anynumber of alternating single and double bonds and aromatic rings,including functional groups on the aromatic rings. The requirement isthat the precursors are hindered from rotation around internal linkages,so as to maintain the largest dipole moment that reverses uponexcitation with actinic light, and that the precursors can beconjugatively linked to form the final chromophore.

The result of the first and second reactions 110, 125 is to provide amonolayer of the voltage sensitive chromophore that is covalently boundto the surface 100 (i.e., the voltage sensitive chromophore surface130). In accordance with the method of FIG. 1, a third reaction 150 isalso performed to form a voltage sensitive chromophore 152 (FIG. 2C),which is dissolved in a liquid 132 to provide a voltage sensitivechromophore solution 155.

As illustrated in FIG. 2C, in an exemplary embodiment the third reaction150 is performed by reacting the voltage sensitive chromophore precursor120 with chromophore component 145 (which is the same chromophorecomponent 145 that was included in the linking group 105 (see FIG. 2A)),thereby forming the voltage sensitive chromophore 152. The voltagesensitive chromophore 152 is dissolved in a sample of the liquid 132, toprovide the voltage sensitive chromophore solution 155. In some cases,the third reaction 150 is carried out in the liquid 132 to directlyprovide the voltage sensitive chromophore solution 155. In other cases,the third reaction 150 can be carried out in another solvent, and thevoltage sensitive chromophore 152 can then be dried and re-dissolved inthe liquid 132 to provide the voltage sensitive chromophore solution155. Examples of appropriate liquids 132 that can be used would includewater, methanol, ethanol, chloroform, acetone, toluene and methylchloride. In general, any liquid that does not react chemically with thevoltage sensitive chromophore surface 130 to break the covalent bondbetween the voltage sensitive chromophore 152 and the surface 100 anddoes not change the conjugated pi system of the voltage sensitivechromophore 152 can be used in accordance with the present invention.For the method of FIG. 1, another requirement is that the voltagesensitive chromophore 152 be soluble in the liquid 132.

After, the voltage sensitive chromophore surface 130 and the voltagesensitive chromophore solution 155 have been prepared, fluorescenceemission spectra are measured and compared. A measure surfacefluorescence emission spectrum step 135 is used to measure a surfacefluorescence emission spectrum 140 of the voltage sensitive chromophoresurface 130 while it is in contact with a sample of the liquid 132. Thisinvolves irradiating the voltage sensitive chromophore surface 130 withactinic radiation provided by an irradiation source while it is incontact with the liquid and using a fluorescence sensing system tomeasure the emission spectrum of the resulting fluorescent radiation. Inthis context, “actinic radiation” is radiation that is adapted tostimulate fluorescence of the voltage sensitive chromophore. Typically,the actinic radiation for many voltage sensitive chromophores will beultraviolet (UV) radiation or short wavelength visible radiation. Forexample, in the case of the exemplary voltage sensitive chromophore 152of FIGS. 2B-2C, the actinic radiation can be radiation having awavelength of 480 nm. In some configurations, the liquid 132 can bebrought into contact with the voltage sensitive chromophore surface 130by using a fluid cell (e.g., a cuvette), where the voltage sensitivechromophore surface 130 is used for one or more surfaces of the fluidcell. Similarly, a measure solution fluorescence emission spectrum step160 is used to measure a solution fluorescence emission spectrum 165 ofthe voltage sensitive chromophore solution 155. This can be accomplishedby using a fluid cell having transparent windows filled with the voltagesensitive chromophore solution 155.

In some configurations, the solution fluorescence emission spectrum 165can be predetermined and stored for later comparison with the surfacefluorescence emission spectrum 140. For example, a number of differentsurfaces 100 can be evaluated using the method of FIG. 1. Voltagesensitive chromophore surfaces 130 can be prepared, each having amonolayer of the voltage sensitive chromophore. Surface fluorescenceemission spectrum 140 can then be measured for each of the surfaces 100,and can be compared to the stored solution fluorescence emissionspectrum 165 for the voltage sensitive chromophore solution 155 ratherthan repeating the measurement of the solution fluorescence emissionspectrum 165.

Once the surface fluorescence emission spectrum 140 and the solutionfluorescence emission spectrum 165 are determined, they are comparedusing a determine interfacial electric field intensity step 170 todetermine a measured interfacial electric field intensity 175. As willbe discussed below, in an exemplary arrangement, the interfacialelectric field intensity 175 is determined responsive to a Stark shiftin the peaks of the fluorescence emission spectra.

FIG. 3A shows a graph 300 comparing measured fluorescence emissionspectra for the case where the exemplary voltage sensitive chromophore152 of FIGS. 2B-2C is used with a quartz surface 100, and the liquid 132(FIG. 1) is water. The fluorescence emission spectra were measured usinga Spex-JY-Horiba Tau 3 spectrometer with a double monochromator on boththe excitation and emission side, and a Peltier-cooled photomultipliertube detector. To measure the surface fluorescence emission spectrum140, a quartz cell (i.e., a cuvette) having a monolayer of the voltagesensitive chromophore 152 on the inside surface was prepared byperforming the first and second reactions 110, 125 in the cuvette. Thecuvette was dried, and then filled with the liquid 132 and irradiatednormal to the surface 100 with actinic radiation having an excitationwavelength of 480 nm, and the resulting emission spectrum was measuredat an angle of 22°. To measure the solution fluorescence emissionspectrum 165, the voltage sensitive chromophore solution 155 wasprepared by dissolving 1 mg of the voltage sensitive chromophore 152 in15 g of water. This solution was diluted by adding 1 ml of the solutionto 19 ml of water (a 1/20 dilution). A cuvette was then filled with thevoltage sensitive chromophore solution 155, and the solutionfluorescence emission spectrum 165 was measured as before using the sameexcitation wavelength (480 nm).

It can be seen that the surface fluorescence emission spectrum 140measured for the voltage sensitive chromophore surface 130 (FIG. 1)having a monolayer of the voltage sensitive chromophore 152 has beenshifted toward a higher wavenumber (i.e., to a higher energy level)relative to the solution fluorescence emission spectrum 165 measured forthe voltage sensitive chromophore solution 155 (FIG. 1). In this case,the Stark shift between the fluorescence emission spectra (i.e., thechange in the wave number between the spectral peeks) was determined tobe Δ{tilde over (ν)}=260 cm⁻¹.

FIG. 3B shows an analogous graph 310 comparing measured fluorescenceemission spectra for the case where the liquid is methanol. In thiscase, the shift between the fluorescence emission spectra was determinedto be Δ{tilde over (ν)}=482 cm⁻¹. Similarly, FIG. 3C shows a graph 320comparing measured fluorescence emission spectra for the case where theliquid is acetone. In this case, the shift between the fluorescenceemission spectra was determined to be Δ{tilde over (ν)}=687 cm⁻¹.

Those skilled in the art will recognize that it will typically bedesirable to smooth the measured fluorescence emission spectra using anappropriate smoothing algorithm to smooth out any measurement noise. Anyappropriate curve smoothing algorithm known in the art can be used inaccordance with the present invention. Example curve smoothingalgorithms would include low-pass filter algorithms, smoothing splinealgorithms, and curve-fitting algorithms that fit a standard functionalform (e.g., a Gaussian function) to the measured data. A peak-findingalgorithm can then be used to identify the peaks of the fluorescenceemission spectra in order to determine the Stark shift. Peak-findingalgorithms are well-known in the data processing art, and anyappropriate algorithm can be used in accordance with the presentinvention.

FIG. 4 is a schematic showing a molecule of the voltage sensitive dye152 bonded to a silica surface 100. The voltage sensitive chromophore152 has an associated dipole moment {right arrow over (P)} and thesurface 100 has an associated interfacial electric field {right arrowover (E)}, with θ being the angle between the {right arrow over (P)} and{right arrow over (E)} vectors. The voltage sensitive dye 152 can rotatearound transperiplanar rotomer 340 to align with the {right arrow over(E)} field.

FIG. 5A is a schematic showing a monolayer of the voltage sensitivechromophore 152 having an associated dipole moment {right arrow over(P)} bound to surface 100 in the presence of an interfacial electricfield {right arrow over (E)}. Upon illumination by actinic radiation(hν_(a)), there is a displacement of charge within the voltage sensitivechromophore 152, causing the dipole moment {right arrow over (P)} to beflipped from ground state 350 to exited state 360.

FIG. 5B is an energy diagram showing the ground state energy level 352associated with the ground state 350 (FIG. 5A) and the excited stateenergy level 362 associated with the excited state 360 (FIG. 5A). As thevoltage sensitive chromophore 152 falls from the exited state 360 backto the ground state 350, a photon is emitted having an energyΔE_(sensor) given by:

ΔE _(sensor) =hc{tilde over (ν)} _(sensor)  (1)

where h=6.626×10⁻³⁴ J·s is Planck's constant, c=3.00×10⁸ m/s is thespeed of light, and {tilde over (ν)}_(sensor) is the wave number(1/wavelength) of the emitted photon.

The excited state energy level 364 associated with the voltage sensitivechromophore 152 when it is in solution is lower than the excited stateenergy level 362 for the sensor where the voltage sensitive chromophore152 is bound to the surface 100, such that the emitted photons have alower energy ΔE_(solution) given by:

ΔE _(solution) =hc{tilde over (ν)} _(solution)  (2)

where {tilde over (ν)}_(solution) is the wave number of the emittedphoton.

The difference between the energies of the excited state energy levels362, 364 (ΔE_(interface)) corresponds to the Stark shift:

$\begin{matrix}\begin{matrix}{{\Delta \; E_{interface}} = {{\Delta \; E_{sensor}} - {\Delta \; E_{solution}}}} \\{= {{{hc}{\overset{\sim}{v}}_{sensor}} - {{hc}{\overset{\sim}{v}}_{solution}}}} \\{= {{hc}\; \Delta {\overset{\sim}{v}}_{Stark}}}\end{matrix} & (3)\end{matrix}$

where the Stark shift Δ{tilde over (ν)}_(Stark)={tilde over(ν)}_(sensor)−{tilde over (ν)}_(solution) is the difference between thewave numbers of the emitted photons in the sensor and solutionconfigurations. The Stark shift can also be related to a correspondingdifference in the wavelengths:

$\begin{matrix}\begin{matrix}{{\Delta {\overset{\sim}{v}}_{Stark}} = {{\overset{\sim}{v}}_{sensor} - {\overset{\sim}{v}}_{solution}}} \\{= {{1/\lambda_{sensor}} - {1/\lambda_{solution}}}} \\{= {\left( {\lambda_{solution} - \lambda_{sensor}} \right)/\left( {\lambda_{sensor} \cdot \lambda_{solution}} \right)}}\end{matrix} & (4)\end{matrix}$

where λ_(sensor) and λ_(solution) are the wavelengths of the emittedphotons in the sensor and solution configurations, respectively.

The energy difference ΔE_(interface) corresponds to the energydifference between the two orientations of the chromophore dipole in thepresence of the interfacial electric field, which is directly related tothe magnitude of interfacial electric field {right arrow over (E)} andthe chromophore dipole moment {right arrow over (P)}, and the angle θbetween them:

ΔE _(interface)=2|{right arrow over (E)}∥{right arrow over (P)}|cosθ  (5)

Combining Eqs. (3) and (5) and solving for the magnitude of theinterfacial electric field {right arrow over (E)} gives:

$\begin{matrix}{{\overset{->}{E}} = \frac{{hc}\; \Delta {\overset{\sim}{v}}_{Stark}}{2{\overset{->}{P}}\cos \; \theta}} & (6)\end{matrix}$

Therefore, experimentally measuring the Stark shift Δ{tilde over(ν)}_(Stark) enables the magnitude of the interfacial electric field|{right arrow over (E)}| to be determined given estimates of the dipolemoment and the angle that the chromophore dipole makes with theinterfacial electric field at the interface. The cosine function isrelatively close to 1.0 for a fairly large range of angles around θ=0°,so that it can be neglected in many cases without significantlyimpacting the calculated result. Even if θ is as large as 60°, thecalculation of the electric field would only be in error by a factor ofabout 2×. Note that any solvatochromic shift associated with theparticular liquid is nulled out given that both spectra are taken in thesame liquid.

The dipole moment of the voltage sensitive chromophore 152 of FIGS.2B-2C, which was used to produce the experimental results of FIGS. 3A-3Cwas estimated to be:

{right arrow over (P)}=e·δ/2=(1.60×10⁻¹⁹ C)(0.217 nm)/2=1.74×10⁻²⁹C·m  (7)

where e=1.60×10⁻¹⁹ C is the elementary charge, and δ=0.217 nm is theestimated distance that the charge is displaced when the polarity of thedipole is flipped. Using this value, the magnitudes of the interfacialelectric fields for each case can be calculated using Eq. (6). Theresults are summarized in Table 1, where the dipole angle was estimatedto be θ=45°.

TABLE 1 Measured interfacial electric fields Surface Liquid Δ{tilde over(ν)} (cm⁻¹) |{right arrow over (E)}| (V/cm) quartz water 260 2.11 × 10⁶quartz methanol 482 3.90 × 10⁶ quartz acetone 687 5.56 × 10⁶

From Table 1, it can be seen that for a given surface (e.g., quartz witha monolayer of a particular voltage sensitive chromophore 152), themagnitude of the interfacial electric field, and therefore the Starkshift, vary according to the characteristics of the liquid. In someembodiments this fact can be used to provide a sensor for determiningcharacteristics of an unknown liquid. One of the primary liquidcharacteristics that will influence the interfacial electric field isthe dielectric constant of the liquid. FIG. 6 shows a graph 400 of theStark shift Δ{tilde over (ν)}_(Stark) as a function of the dielectricconstant, ∈_(r), using the data from Table 1. A curve 410 has been fitto the measured data, giving a relationship between the dielectricconstant and the resulting Stark shift. If the Stark shift is measuredfor an unknown liquid, the curve 410 can be used to determine anestimate of the dielectric constant of the liquid.

FIG. 7 shows a flow chart of a method for determining a characteristicof a liquid in accordance with an embodiment of the invention. Themethod of FIG. 7 is identical to that of FIG. 1, except that rather thananalyzing the surface and solution fluorescence emission spectra 140,165 to determine the interfacial electric field intensity 175, adetermine characteristic of liquid step 200 is used to determine aliquid characteristic 205.

In an exemplary arrangement, the determine characteristic of liquid step200 computes the Stark shift, and then uses the curve of FIG. 6 todetermine a dielectric constant of the liquid 132. For the case wherethe liquid 132 is an unknown substance, the determine characteristic ofliquid step 200 can be used to identify the liquid, or to determine oneor more candidate identities for the liquid. For example, a table ofdielectric constants for known liquids can be prepared, and the measureddielectric constant for the unknown liquid can be compared to the tableto identify candidate liquids having dielectric constants that match themeasured dielectric constant to within the associated measurement error.This approach can be used to construct sensors capable of identifying orcharacterizing liquids without the use of complexing agents or selectivemembranes as is generally required in the prior art. Other examples ofliquid characteristics 205 that could be determined in a similar fashionin accordance with the invention would include the polarizability andelectric susceptibility.

In some embodiments, the determine characteristic of liquid step 200determines the liquid characteristic 205 based on only the surfacefluorescence emission spectrum 140, without measuring the solutionfluorescence emission spectrum 165. For example, surface fluorescenceemission spectra 140 can be measured for a library of common liquidsusing the voltage sensitive chromophore surface 130. A surfacefluorescence emission spectrum 140 can then be measured for an unknownliquid 132. The resulting surface fluorescence emission spectrum 140 forthe unknown liquid can then be compared to the library of surfacefluorescence emission spectra 140 for the common liquids to determinewhether there is a match. If so, it can be determined that the unknownliquid is likely to be the matching liquid from the library. In thisexample, the determined characteristic 205 is the identity of the liquid132.

FIG. 8 shows a flow chart of a method for determining a characteristicdifference between different liquids in accordance with an embodiment ofthe invention. The method of FIG. 8 is similar to that of FIG. 1, exceptthat rather than comparing the fluorescence emission spectrum determinedfrom the voltage sensitive chromophore surface 130 with that measuredfor the voltage sensitive chromophore solution 155, the voltagesensitive chromophore surface 130 is used to compare a first liquid 210and a second liquid 215. In this case, a measure first fluorescenceemission spectrum step 220 provides a first fluorescence emissionspectrum 225 for the first liquid 210 in contact with voltage sensitivechromophore surface 130, and a measure second fluorescence emissionspectrum step 230 provides a second fluorescence emission spectrum 235for the second liquid 215 in contact with the voltage sensitivechromophore surface 130. A characterize difference between liquids step240 is then used to compare the first and second fluorescence emissionspectra 225, 235 to determine a difference between the liquids 245. Forexample, the difference between the liquids 245 can be a parameterrepresenting the degree of similarity (or difference) between the firstand second fluorescence emission spectra 225, 235 (e.g., a correlationcoefficient, or an RMS difference between the spectra). Alternatively,the difference between the liquids 245 can be a parameter indicating thelikelihood that the first liquid 210 and the second liquid 215 are thesame (e.g., a statistical confidence level). In other arrangements, thedifference between the liquids 245 can be one or more parameterscharacterizing the difference between the first and second fluorescenceemission spectra 225, 235 (e.g., a difference between the peakwavelengths, or a difference function representing the difference as afunction of wavelength).

A variety of applications can be envisioned for the method of FIG. 8.For example, a distillation process can be used to extract a particularliquid from a mixture of liquids. During the distillation process, thepurity of the liquid will gradually increase. In this case, the firstliquid 210 can be a reference sample of liquid having a desired level ofpurity, and the second liquid 215 can be a sample of the distillationproduct at a particular point in time. The difference between theliquids 245 can be determined at a series of times, and the distillationprocess can be terminated when it is determined that the differencebetween the liquids 245 falls below a predefined threshold indicatingthat the distillation product matches the reference sample.

While the method of FIG. 8 was described with respect to comparing firstand second liquids 210, 215, one skilled in the art will recognize thatthe method can be generalized to compare two fluids, where one or bothof the fluids can be gases.

The determine interfacial electric field intensity step 170 of FIG. 1,the determine characteristic of liquid step 200 of FIG. 7 and thecharacterize difference between liquids step 240 of FIG. 8 generallyinvolve making a comparison between a pair of fluorescence emissionspectra. The comparison methods described above have involveddetermining a Stark shift by comparing the positions of the peaks of thefluorescence emission spectra (either in terms of wave numberdifferences or wavelength differences). In other arrangements, differentmethods can be used to compare the two fluorescence emission spectra.For example, in one variation, rather than comparing the peaks of thefluorescence emission spectra, centroids (or other central tendencymetrics) can be determined for each fluorescence emission spectra andcompared. Other statistical metrics can also be used to characterize thefluorescence emission spectra, including standard deviation, skew andkurtosis.

FIG. 9 is a graph illustrating exemplary first and second fluorescenceemission spectra 225, 235. In this case, the first surface fluorescenceemission spectrum 225 was determined for a first liquid 210 (water), andthe second fluorescence emission spectrum 225 was determined for asecond liquid 215 (methanol), in accordance with the method of FIG. 8.

In an exemplary class of spectrum comparison methods, a first wavelength430 and a second wavelength 435 are defined. In the illustrated example,the first wavelength 430 is 650 nm and the second wavelength 435 is 560nm. Preferably, the first and second fluorescence emission spectra 225,235 overlap with both the first and second wavelengths 430, 435. A firstintensity ratio R1=A1/B1 is determined for the first fluorescenceemission spectrum 225, where A1 is the value of the first fluorescenceemission spectrum 225 at the first wavelength 430, and B1 is the valueof the first fluorescence emission spectrum 225 at the second wavelength435. Likewise, a second intensity ratio R2=A2/B2 is determined for thesecond fluorescence emission spectrum 235, where A2 is the value of thesecond fluorescence emission spectrum 235 at the first wavelength 430,and B2 is the value of the second fluorescence emission spectrum 235 atthe second wavelength 435.

If a fluorescence emission spectrum is centered between the first andsecond wavelengths 430, 435, then the intensity ratio will beapproximately equal to 1.0. If the spectrum is to the left of center,then the intensity ratio will be smaller than 1.0, and if the spectrumis to the right of center, then the intensity ratio will be greater than1.0. Therefore, the intensity ratio will be a measure of the location ofthe spectral peak for the spectrum, and the difference in the intensityratios will be a measure of the shift in the spectral peaks.

Table 2 shows exemplary ratios determined for the fluorescence emissionspectra of FIG. 9. It can be seen that the difference between spectralpeaks is clearly reflected by differences in the intensity ratios. Thelarger intensity ratio for water reflects the fact that its emissionspectrum (first fluorescence emission spectrum 225) is shifted to theright relative to that for methanol (second fluorescence emissionspectrum 235).

TABLE 2 Intensity ratio comparison A (λ = 650 nm) B (λ = 560 nm) R = A/BWater 0.680 0.349 1.95 Methanol 0.510 0.690 0.74

In some embodiments, the intensity ratios can be related to the quantitybeing measured (e.g., the interfacial electric field intensity 175(FIG. 1) or the liquid characteristic 205 (FIG. 7)) by evaluating theintensity ratios for a set of reference configurations and fitting afunction to the results.

In other types of spectrum comparison methods more than two wavelengthscan be compared. For example, additional spectra ratios can be computedbetween additional wavelengths pairs to provide additional informationabout the shape and position of the spectrum. In some cases, a spectraldifference can be determined by subtracting the two spectra on awavelength-by-wavelength basis. The spectral difference can then beanalyzed to characterize the difference between the spectra. Forexample, an RMS spectral difference can be computed between the spectrameasured for two liquids to determine if the spectra are statisticallyindistinguishable, indicating that the two liquids may be the same. Insome configurations, the spectra can be analyzed to determinecorresponding color values (e.g., in the well-known L*a*b* color space),and color differences can be computed (e.g., ΔE* values) to characterizethe difference between two spectra.

In some embodiments, many spectra are compared to determine which liquidis present at the surface. In such cases, any classification anddiscrimination analysis methods known in the statistics and chemometricsfields can be used. These methods include the use of the Mahalanobisdistance, K nearest neighbor, discriminant analysis, cluster analysis,factor analysis, and principal component analysis (PCA) and obtainingspectral contrast angles between pairs of spectra. As the number ofcompared spectra increases, the more useful these methods become,especially PCA and cluster analysis, leading to full pattern recognitionof the spectral shape of a given liquid/chromophore spectral response.Many of these techniques can be extended to spectral modeling ordeconvolution, allowing a liquid to be identified by all of itswavelength responses.

FIG. 10 shows an exemplary surface evaluation system 500 for evaluatinga surface. A chromophore application system (not shown) is used to applya monolayer of a voltage sensitive chromophore 152 (FIG. 2B) to asurface 100 (FIG. 2B) to provide voltage sensitive chromophore surface130. The applied voltage sensitive chromophore 152 is covalently boundto the surface 100 and has a fluorescence emission spectrum which variesin accordance with a characteristic of the surface 100 (e.g., theinterfacial electric field). The voltage sensitive chromophore surface130 is mounted in a surface holder 505 for measurement. One skilled inthe art will recognize that a wide variety of arrangements can be usedfor the surface holder 505 in accordance with the present invention. Inthe illustrated configuration, the surface holder 505 includes arecessed surface that is adapted to receive a liquid 132 in someembodiments. In other configurations, the voltage sensitive chromophoresurface 130 can be a surface of a liquid cell (e.g., a cuvette), and thesurface holder 505 can be a mechanism for holding the liquid cell.

An irradiation source 510 is positioned to irradiate the voltagesensitive chromophore surface 130 with actinic radiation that stimulatesfluorescence of the voltage sensitive chromophore 152. A fluorescencesensing system 515 (e.g., a spectrophotometer) measures a surfacefluorescence emission spectrum 140 which is emitted by the irradiatedvoltage sensitive chromophore surface 130. In some embodiments, thefluorescence sensing system 515 is a micro-spectrophotometer which isadapted to sense the surface fluorescence emission spectrum 140 emittedfrom a localized area voltage sensitive chromophore surface 130. In thiscase, the irradiation source 510 can illuminate the same localized area(as shown in FIG. 10), or alternatively can illuminate a broad area ofthe voltage sensitive chromophore surface 130.

Generally, the surface fluorescence emission spectrum 140 will bemeasured while the voltage sensitive chromophore surface 130 is incontact with some fluid (e.g., a liquid or a gas). In some embodiments,the surface fluorescence emission spectrum 140 can be measured while thevoltage sensitive chromophore surface 130 is exposed to air (or someother gas). In other embodiments, the surface fluorescence emissionspectrum 140 can be measured while the voltage sensitive chromophoresurface 130 is in contact with a liquid 132. In this case, a liquidapplicator (not shown) can be used to bring the liquid 132 into contactwith the voltage sensitive chromophore surface 130. For example, theliquid 130 can be added into the recessed region in the surface holder505 of FIG. 10. For the case where the voltage sensitive chromophoresurface 130 is the wall of a liquid cell, the liquid applicator can addthe liquid 132 to the liquid cell. Depending on the geometry of thevoltage sensitive chromophore surface 130 and the other components ofthe surface evaluation system 500, one skilled in the art will recognizethat a wide variety of liquid applicators can be used to bring theliquid 132 into contact with the voltage sensitive chromophore surface130.

An analysis system 520 is used to analyze the surface fluorescenceemission spectrum 140 to evaluate a characteristic of the voltagesensitive chromophore surface 130. In an exemplary embodiment, thecharacteristic is the interfacial electric field intensity 175 (FIG. 1)of the voltage sensitive chromophore surface 130. The analysis stepsinvolved with the determination of the interfacial electric fieldintensity 175 have been discussed above. In some embodiments, thesolution fluorescence emission spectrum 165 of the voltage sensitivechromophore solution 155 can be predetermined and stored as a referencefluorescence emission spectrum for comparison with the measured surfacefluorescence emission spectrum 140 at a later time. In otherembodiments, the analysis system 520 can be used to determine a liquidcharacteristic 205 as discussed with respect to FIG. 7, or to determinedifferences between a plurality of liquids, as was discussed withrespect to FIG. 8.

In some embodiments, the fluorescence sensing system 515 measures aplurality of surface fluorescence emission spectra 140 at a lattice ofspatial positions 525 on the voltage sensitive chromophore surface 130.In this case, either the surface holder 505 or the fluorescence sensingsystem 515 can be moved in a scanning path in order to sense the surfacefluorescence emission spectra 140 at the different positions. In suchconfigurations, the analysis system 520 can be used to analyze theplurality of surface fluorescence emission spectra 140 to evaluate auniformity of a characteristic of the voltage sensitive chromophoresurface 130. This can be useful for a number of applications, includingdetecting surface defects or inspecting features on the surface.

There are a wide variety of applications that involve depositing apattern of features onto a surface, including the fabrication ofelectrical components or devices (e.g., on a silicon wafer or on a glassor polymer substrate). In some cases, it can be difficult to inspectsuch components, particularly when they may include features that arefabricated using transparent, or partially transparent, materials. Suchfeatures may not be easily detected by conventional machine visioninspection techniques that involve capturing an image of the surfaceusing visible light. However, the presence of the features will producelocal perturbations in the interfacial electric field of the surface.These perturbations can be detected using methods in accordance withembodiments of the present invention by applying a monolayer of voltagesensitive chromophore to the surface and measuring the fluorescenceemission spectrum. The presence of edges and surface textures on thesurface can also affect the amplitude and direction of the emittedfluorescent light. Such perturbations can be detectable with appropriatetypes of sensors.

In one such embodiment, the surface 100 having the features thereon istreated to attach a monolayer of the voltage sensitive chromophore 152(FIG. 2B) providing the voltage sensitive chromophore surface 130 asdescribed above relative to FIG. 1. The surface evaluation system 500can then be used to measure a pattern of fluorescent light as a functionof spatial position on the voltage sensitive chromophore surface 130. Inan exemplary configuration, the pattern of fluorescent light can becharacterized by measuring the surface fluorescence emission spectra 140at a lattice of spatial positions 525 as described above. In otherconfigurations, the pattern of fluorescent light can be characterized bymeasuring an intensity or a color (or some other attribute) of thefluorescent light. In such cases, the fluorescence sensing system 515does not necessarily need to determine the surface fluorescence emissionspectra 140, but rather can directly measure the fluorescent lightcharacteristic using other types of measurements (e.g., intensitymeasurements or color measurements).

The analysis system 520 can analyze the measured pattern of fluorescentlight in order to inspect the surface. In some embodiments, features inthe measured pattern of fluorescent light can be compared to a referencepattern associated with the device being fabricated, for example toverify that there are no shorts or voids in a pattern of micro-wires. Insome embodiments, the measured pattern of fluorescent light can beanalyzed to detect surface defects (e.g., non-uniformities in a surfaceregion that was supposed to be uniform).

When the surface evaluation system 500 is being used to measure apattern of fluorescent light, it may not be necessary to measure thefluorescent light while the voltage sensitive chromophore surface 130 isin contact with a liquid 132, particularly if the goal is to detectnon-uniformities or other defects rather than determining quantitativeinterfacial electric field intensities. In such cases, the pattern offluorescent light can be measured when the voltage sensitive chromophoresurface 130 is exposed to air (or some other gas).

The methods described herein have many advantages compared to othermethods for using voltage sensitive chromophores currently used in theart. The describes methods provide a chromophore that is covalentlyattached to the surface. In contrast, other methods, such as thosedescribed in the aforementioned articles by Loew and Pope et al., usechromophores embedded in membranes or self-assembled monolayers. In sucharrangements, the location of the chromophore is not known precisely,nor is the measured fluorescence emission spectra guaranteed to be froma chromophore in a known position with respect to the surface ormembrane, but can be mixed with the spectra of unembedded chromophoremolecules and chromophore molecules at various distances from thesurface. In the methods of the present invention, the voltage sensitivechromophore is attached to the surface in a known way, and is createdonly when it is attached to the surface, so the position of thechromophore is known precisely.

Another advantage of the present invention is that it can be used with awide variety of different surfaces including silica, polymer, metal,inorganic, and other surfaces. Other methods have restrictions on thesample surface, such as requiring a biological membrane a metal surfaceor a metal electrode. For example, the method described in theaforementioned articles by Pope et al. require that the surface be ametal electrode, and specifically a silver electrode, in order toenhance the fluorescence of the chromophore. Our method has been shownto work without metal-enhanced fluorescence.

In other methods, such as those described in the aforementioned articlesby Pope et al., external fields need to be applied (for example, to anelectrode surface or a membrane surface) to provide a change in theelectric field to induce a Stark shift, which is measured by a shift inthe fluorescence emission spectrum. In methods of the present invention,the emission of the chromophore on the surface is compared to theemission of the chromophore in solution, which enables the Stark shiftto be determined and the interfacial electric field to be calculatedwithout the use of any applied field. This makes the measurementdependent only on the voltage sensitive chromophore, the surface, andthe liquid, and is not dependent on any applied fields.

Because the methods in this application do not require applied fields,the methods are simpler and do not require the use of more complicatedinstrumentation. Additionally, the methods can be applied over a largearea, and need not be confined to smaller areas, such as in atomic forcemicroscopy.

Furthermore, the methods are non-destructive for the sample, and onlyrequire that the surface be modified to include the voltage sensitivechromophore. In other methods for determining an interfacial electricfield, such as the streaming potential method described in theaforementioned article by Xie et al. and the atomic force microscopymethod described in the aforementioned article by Li et al., the sampleis significantly changed, and may not be used for any other testing.

Generally, the methods of the present invention are simpler, moreprecise, more accurate, and more versatile than prior art methods formeasuring interfacial electric fields. Furthermore, the present methodscan be used to characterize large areas of a surface, and to provide asensor for characterizing a liquid.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   100 surface-   105 linking group-   110 first reaction-   112 covalent linkage-   115 surface with linking group-   120 voltage sensitive chromophore precursor-   125 second reaction-   130 voltage sensitive chromophore surface-   132 liquid-   135 measure surface fluorescence emission spectrum step-   140 surface fluorescence emission spectrum-   145 chromophore component-   147 coupling agent-   150 third reaction-   152 voltage sensitive chromophore-   155 voltage sensitive chromophore solution-   160 measure solution fluorescence emission spectrum step-   165 solution fluorescence emission spectrum-   170 determine interfacial electric field intensity step-   175 interfacial electric field intensity-   180 p-substituted dialkylamino aryl group-   182 conjugate linkage-   185 N-containing heterocyclic aromatic group-   200 determine characteristic of liquid step-   205 liquid characteristic-   210 first liquid-   215 second liquid-   220 measure first fluorescence emission spectrum step-   225 first fluorescence emission spectrum-   230 measure second fluorescence emission spectrum step-   235 second fluorescence emission spectrum-   240 characterize difference between liquids step-   245 difference between the liquids-   300 graph-   310 graph-   320 graph-   340 transperiplanar rotomer-   350 ground state-   352 ground state energy level-   360 exited state-   362 exited state energy level-   364 exited solution state energy level-   400 graph-   410 curve-   420 graph-   430 first wavelength-   435 second wavelength-   500 surface evaluation system-   505 surface holder-   510 irradiation source-   515 fluorescence sensing system-   520 analysis system-   525 lattice of spatial positions

1. A method for determining a characteristic difference between first and second fluids, comprising: providing a surface having a reactive carbocyclic aromatic linking group covalently attached thereon; providing a voltage sensitive chromophore precursor including a p-substituted dialkylamino aryl group that is conjugatively linked to a terminal N-containing heterocyclic aromatic group; reacting the voltage sensitive chromophore precursor with the reactive carbocyclic aromatic linking group that is covalently attached to the surface to form a monolayer of a voltage sensitive chromophore that is covalently bound to the surface; bringing the first fluid into contact with the monolayer of the covalently bound voltage sensitive chromophore; irradiating the monolayer of the covalently bound voltage sensitive chromophore with actinic radiation while it is in contact with the first fluid and measuring a first fluorescence emission spectrum; bringing the second fluid into contact with the monolayer of the covalently bound voltage sensitive chromophore; irradiating the monolayer of the covalently bound voltage sensitive chromophore with actinic radiation while it is in contact with the second fluid and measuring a second fluorescence emission spectrum; and comparing the first and second fluorescence emission spectra to characterize a difference between the first and second fluids.
 2. The method of claim 1, wherein the reactive carbocyclic aromatic linking group includes a benzyl halide group.
 3. The method of claim 1, wherein the N-containing heterocyclic aromatic group is a pyridinyl group.
 4. The method of claim 1, wherein the voltage sensitive chromophore is a reaction product where the reactive carbocyclic aromatic linking group is bonded to the terminal N-containing heterocyclic aromatic group of the voltage sensitive chromophore precursor.
 5. The method of claim 1, wherein the difference between the first and second fluids is characterized by a parameter representing a degree of similarity between the first and second fluorescence emission spectra.
 6. The method of claim 1, wherein the difference between the first and second fluids is characterized by a parameter indicating the likelihood that the first and second liquids are the same.
 7. The method of claim 1, wherein comparing the first and second fluorescence emission spectra includes: determining a first spectral peak for the first fluorescence emission spectrum; determining a second spectral peak for the second fluorescence emission spectrum; and wherein the characteristic difference between the first and second fluids is determined responsive to a wavelength difference or a wave number difference between the first and second spectral peaks.
 8. The method of claim 1, wherein comparing the first and second fluorescence emission spectra includes: specifying first and second wavelengths; determining a first intensity ratio between a value of the first fluorescence emission spectrum at the first wavelength and a value of the first fluorescence emission spectrum at the second wavelength; determining a second intensity ratio between a value of the second fluorescence emission spectrum at the first wavelength and a value of the second fluorescence emission spectrum at the second wavelength; and wherein the characteristic difference between the first and second fluids is determined responsive to the first and second intensity ratios.
 9. The method of claim 1, wherein the surface having the reactive carbocyclic aromatic linking group covalently attached thereon is formed by: providing a solution including the reactive carbocyclic aromatic linking group; providing a surface capable of reacting with the reactive aromatic linking group; and bringing the solution into contact with the surface, thereby reacting the reactive carbocyclic aromatic linking group with the surface to attach the reactive carbocyclic aromatic linking group to the surface with a covalent linkage.
 10. The method of claim 1, wherein the fluid is water, methanol, ethanol or chloroform.
 11. The method of claim 1, wherein the surface is a silica surface or a polymer surface.
 12. The method of claim 1, wherein the surface is non-biological.
 13. A method for determining a characteristic difference between first and second fluids, comprising: providing a surface having a monolayer of a voltage sensitive chromophore that is covalently bound to the surface, wherein the voltage sensitive chromophore has a fluorescence emission spectrum that varies in accordance with an electric field; bringing the first fluid into contact with the monolayer of the covalently bound voltage sensitive chromophore; irradiating the monolayer of the covalently bound voltage sensitive chromophore with actinic radiation while it is in contact with the first fluid and measuring a first fluorescence emission spectrum; bringing the second fluid into contact with the monolayer of the covalently bound voltage sensitive chromophore; irradiating the monolayer of the covalently bound voltage sensitive chromophore with actinic radiation while it is in contact with the second fluid and measuring a second fluorescence emission spectrum; and comparing the first and second fluorescence emission spectra to characterize a difference between the first and second fluids. 